Structural Health Monitoring and Model Correlation of Advanced Space Systems

THE DETERMINATION OF LOCATION and extent of structural damage and modeling errors are of significant importance in many engineering systems ranging from advanced aerospace structures such as the International Space Station, the Space Shuttle, commercial and military aircraft, and future reusable launch vehicles and spacecraft as described in the NASA Millennium initiative, to the nation's civil infrastructure such as bridges, dams, offshore platforms and large buildings. Mathematical models of these critical structures are produced and used to perform rapid redesigns, estimate life and performance, implement control algorithms, and define operational constraints, in addition to locating and quantifying damage. Experimental testing is required to allow these mathematical models to be updated or correlated and then to periodically monitor the structure for damaging changes in the structural health. This year's efforts have centered around the development of technologies to acquire the experimental data from advanced aerospace systems, efficiently analyze the data, and perform structural health monitoring/model correlation.

STRUCTURAL ERRORS—Above. Co PI’s David Zimmerman, UH (left rear) and Michael S. Grygier, NASA-JSC (right front) team with NASA fellow George H. James III (left front) and NASA specialist Timothy T. Cao (right rear) to study structural stress during flight. They hold a collapsible elastic strut designed to support solar panels, whose strength is tested in space with sensors attached to the structure. Corners with tension wires reveal detailed engineering technology.

Project Overview
The objective of this research program is to develop and validate procedures to allow engineers to enhance structural safety, maintainability and performance using vibration measurements, structural modeling and data processing algorithms. The key concept involves utilizing changes in the "vibration signature" to locate and estimate the extent of damage and/or model errors. Although the problems of damage detection and model correlation require different mathematical solutions, both draw heavily on System Identification (SI) technology. This technology seeks to determine the best fit mathematical model directly from experimental data. This experimental model then provides the comparison metrics for the analytical model.

This technology has important utility in a wide variety of structural applications; however, several current NASA programs will push the technology past its currently developed limits. The space shuttle fleet will continue to fly even as specific components approach the end of their design lifetimes and require recertification and/or the implementation of upgrade programs, including structural monitoring. The space station will open a new realm in SI, model correlation, and damage detection as the structure will be large, flexible, and incapable of being ground tested. The X-38 crew return vehicle design must face a wide variety of structural threats including launch loads; orbital damage accumulation in metallic, ceramic, and composite structures; aerodynamic reentry loads; and extreme landing conditions. Advanced inflatable structures such as the Transhab concept will require unique structural models and will be critically dependent upon an effective structural integrity monitoring system. Finally, reusable launch vehicles must have correlated models and structural health monitoring systems in order to safely and economically provide routine access to space. In order to expand the technology to meet these challenges, the following topics have or will be investigated:

• Experimental Testing--although this ISSO project has been instrumental in defining novel sensors needed to overcome the problems involved in testing the International Space Station (ISS), the primary focus of the current year's activities will be the hardware verification and software development for the Laser Dynamic Range Imager (LDRI).
• Data Analysis--the significant developments of Autonomous Identification (AUTO-ID) technology, for semi-automated analysis of dynamically complex data sets, and load-dependent Ritz vector estimation/utilization procedures using frequency domain and ambient data are part of the past and current efforts.
• Interpretation of Results--the technology for localization of modeling errors and damage locations has been enhanced via the development and implementation of coupled reduction/expansion approaches based on matrix disassembly technology.

Experimental Testing
LDRI Sensor Development. One task performed in the first year of this ISSO project involved the assistance in defining non-contacting sensor systems for measuring structural dynamics of ISS structures. One system, called the Scannerless Range Imager,¹ was developed by Sandia National Laboratories. This system was chosen for further development by NASA-JSC, Sandia, and the University of Houston to allow the measurement of structural dynamics time histories. The modified system was renamed the Laser Dynamic Range Imager (LDRI). The LDRI is a video based sensor system designed to provide a range image as well as a reflectance image (similar to a traditional video image). This allows for the calculation of range for each pixel of video image. Since the reflectance image can provide displacements in two directions at each pixel (called cross-plane or X-plane directions), the range information allows three dimensional displacement measurements. Figure 1a provides the reflectance image from a flat plate. Figure 1b provides the associated range image. Figure 2a provides the associated X-plane displacement time histories for a single point in Fig. 1a. Figure 2b provides range time history for the associated point.

The current configuration will allow displacement-based structural dynamics time histories to be generated at one fourth to one times the video sample rate (typically 1/60 of a second). Hence, structural modes with maximum frequencies of 7 to 30 cycles per second (or Hz) will be measurable. The resolution of the system will be .1 to .01 inches of displacement. The completed ISS is expected to have over 1500 modes with frequencies under 7 cycles per second. Furthermore, the majority of the modes will involve high amplitude displacements of the solar arrays. Since these large structures are largely uninstrumented, the LDRI is a perfect sensor for this unique and ground-breaking application.

LDRI Sensor Validation Tests. An Engineering Development Unit (EDU) will be delivered to NASA for further evaluation of the technology during 1998 and 1999. This ISSO project team has taken a lead role in defining the configuration of the EDU and the associated data acquisition software. Additionally, the ISSO project team will have a lead role in defining the development tests, performing the experiments, analyzing the results, and suggesting hardware/software modifications.

LDRI Flight Test and Mission Planning. The Structures and Dynamics branch at NASA-JSC is actively engaged in planning flight tests and preparing for structural dynamics measurement missions using the LDRI sensor. The near-term opportunity involves mounting an LDRI sensor to the underside of a Space Shuttle Payload Bay camera. This unit would then be used to perform structural dynamics measurements on ISS assembly flight 5A in Spring of 1999. This unit would then fly on at least four additional missions to perform ISS structural dynamics experiments. The LDRI is also currently under consideration to fly as a primary payload on the ARECAM free-flying vehicle. This unit is a follow-on to the SPRINT vehicle which was flown on shuttle mission STS-87 in November and December of 1997. A free-flyer implementation of the LDRI will allow measurements of a wide variety of ISS structures without requiring the presence of the Space Shuttle as a platform. This ISSO project team is actively involved in defining mission objectives, assessing the technology readiness, and designing an LDRI system and procedures to perform such missions.

Data Analysis
Ritz Vector Estimation Procedures. Load dependent Ritz vectors have been shown to be more sensitive to some types of damage and structural model errors than traditional mode shapes. These vectors are also a natural set of basis vectors to retain and utilize the information in the dynamic residuals which result from comparing damaged data to an undamaged structural model or when comparing baseline data to an uncorrelated structural model. However, the limiting issue in utilizing Ritz vectors has been that no procedure was available to experimentally determine Ritz vectors from dynamics data. This issue has been rectified by the development of the Ritz Realization Algorithm (RRA).² This new system identification procedure involves the development of a state-space model of the structural system using time impulse response data. The first Ritz vector is the static deflection of the structural system due to a unit applied load at the shaker location. Additional orthogonal vectors are extracted using the inverse iteration and Gram-Schmidt orthogonalization procedures.

The past year's activities in this area have entailed the expansion of this capability to include Ritz Vector estimation directly from Frequency Response Function (FRF) data.³ The RRA utilizes time history data; however, modal data is often stored in the form of FRFs. Therefore, the additional capability to utilize data in this form is most useful. The FRF analysis approach is also currently under investigation as the starting point for developing mass-normalized Ritz vectors. This normalization further constrains the Ritz vectors to retain the mass properties of the system in a mathematically rigorous manner in addition to the stiffness properties.

Also, the ISSO project team has performed an extension of the RRA to handle the ambient data case.⁴ Hence, there exist cases wherein the measurement force input to the system does not have to be measured in order to extract Ritz Vectors. This is most useful in situations such as launch and reentry conditions where the forcing functions are impossible to measure.

AUTO-ID Development. Model correlation and structural damage detection usually require extensive databases which may require several man-months of effort for analysis. This is a common problem faced by many applications including the currently implemented Shuttle Modal Inspection System (SMIS).⁵ Hence, this project has developed a technique to extract modal parameters from measured time histories or frequency response functions with little or no human intervention. This concept, called AUTOmated system IDentification (AUTO-ID),⁶ is depicted in Fig. 3. This technique has already been applied successfully to 45 subsets of the VSA testing data , data sets from the SMIS system, the X-38 crew return vehicle, as well as on-orbit data obtained from the Russian MIR space station. Development of the AUTO-ID capability represents a significant technology advancement. Table 1 provides X-38 modal frequency and modal damping results obtained using AUTO-ID. The data set was produced during a captive carry test as shown in Fig. 4. The aerodynamic input was not measured and an ambient excitation analysis was used. These damping results are the only consistent damping values produced for the X-38 project using the actual operational loading conditions.

Table 1. X-38 Captive Carry AUTO-ID Results

LDRI Analysis Software Production. A primary task during the current year of this ISSO project is the development of analysis software for the LDRI sensor. This complicated software must combine aspects of image processing, photogrammetry, signal processing, modal parameter extraction, data visualization, and model correlation. Hence, the software will be organized around four interconnected sets of modules. Figure 5 provides a schematic of this arrangement with the modules denoted in the colored boxes. The top leg of modules (denoted by LDRI, Relative Range, and Absolute Range) are those dealing with the actual LDRI sensor itself, as well as its operating software, user interfaces, and modes of operations. The bottom leg of modules (denoted by Geometry, Model, and Comparison to Model) are those dealing with the mathematical description of the structure of interest, including the physical geometry, location of any tracking targets, the analytical prediction of the structural response, and data comparison software. The leg of modules on the left hand side of Fig. 5 includes Matlab Translator through Data Visualization, will process and interpret Range and interpret combined Range/Cross-Plane data. The final leg of modules on the right hand side of Fig. 5 include Position Estimation through Target 3-D Extraction, will extract and process all Cross Plane (X-Plane) motion and estimate three dimensional (3-D) motion. Interfaces between the modules are denoted by the ellipses with arrows denoting direction of information flow. Although members of the UH ISSO project team are the final integrators of the software, there are four groups (UH Mechanical Engineering, Sandia National Laboratories Advanced Sensors Department, NASA-JSC Image Processing Branch, and NASA-JSC Structures and Dynamics Branch) which have some responsibility for the individual modules.

Interpretation of Results
Error/Damage Localization. The development of the Space Shuttle Orbiter vehicles in the late 1970s and early 1980s included detailed testing of several significant structural components. The Vertical Stabilizer Assembly (VSA) test article was one of these components. It was used as an acoustic test article from 1979 to 1981 and as a static fatigue test article in 1982. It was used to verify that the static and fatigue response of the shuttle vertical tail met the design requirements. The VSA was recently returned to NASA-JSC for a series of experiments to support development and demonstration of model correlation and damage identification technologies. The assembled structure consists of the upper section of the fin for the vertical stabilizer, the two upper Rudder/ Speed Brakes (RSB), two aluminum actuator mockups, and a steel transition flange (for attachment to ground). The Thermal Protection System (TPS) and all static load application pads had been removed prior to the testing discussed herein. The VSA is identical to the original OV102 (Columbia) vehicle design. Figure 6a shows the operational configuration of the orbiter vehicle with the upper rudder marked. The VSA test article was reassembled in the NASA-JSC Vibration and Acoustic Test Facility. Figure 6b shows the fully assembled VSA as configured for the tests described herein. During the assembly process, modal testing was performed after the addition of each major subassembly. After full assembly, a series of repairable and nonrepairable damage cases was inflicted on the structure. Therefore, an extensive database was produced to study model correlation and damage identification procedures.

The availability of AUTO-ID allowed the rapid processing of all VSA damage detection test data. Figure 7 illustrates the utility of this capability as plots of changes in the modal parameters provide an indication of the onset of significant structural changes over the course of the test series. The upper plot shows the changes experienced in the modal frequency of the modal frequency initially measured at 32.26 Hz as a function of test case. This mode is an out-of-plane rotation of the RSBs. The damping value changes are shown in the second plot. MAC is a scalar measure of the change in a modal shape parameter. Changes in this parameter are provided in the bottom plot of Fig. 7. The first 12 cases are all baseline cases and represent the same nominal structure (with changes due to non-linearities or assembly/disassembly conditions). The modal frequency remains fairly constant throughout the baseline tests. The MAC value and damping values change most radically as the structure is assembled and disassembled as seen in the baseline cases and the repairable damage cases (13-25). These values hold fairly constant during the non-repairable damage cases (26-45). This mode is seen to be relatively sensitive to changes in the rear structural spar as is evident at cases 13-16 (removal of rear spar plates) and cases 39-40 (cuts in rear spar). The relatively significant changes in frequency and damping at case 25 show the sensitivity to removal of the tip cap access panel which was known to be a significant damage case. The large frequency change at damage case 20 reveals the sensitivity of this specific mode to changes in a connecting link between an RSB and an actuator.

Incomplete Measurements Problem. A problem shared with all developed approaches for damage detection and model correlation is that of the incomplete measurement problem. The incomplete measurement problem has two contributions: (1) experimental measurement of a lesser number of modes of vibration than that of the analytical model and (2) experimental measurement of a lesser number of degrees of freedom than that of the analytical model.

One approach to address problem (2) practically is either to reduce the analytical model to the test degrees of freedom or to expand the measured modal data to all degrees of freedom included in the analytical model.⁷ Unfortunately, both of these approaches cause problems when performing damage detection. An observed problem with model reduction is that localized changes in the full model may become "smeared" throughout the reduced model. A problem observed with mode shape expansion is that errors introduced in the expansion process lead to false positive indications of damage. In this work, we extend the theory of Minimum Rank Perturbation Theory (MRPT)⁸ dynamic residuals and coupled the idea with concepts of stiffness matrix disassembly⁹ to arrive at an expanded dynamic residual. The development of an expanded dynamic residual using a Finite Element Model (FEM) of the structure and measured modal data is based on work initially proposed in James et al.¹⁰ This approach has drastically improved the analysis of systems with incomplete measurements. The NASA eight-bay¹¹ experimental results are used to illustrate the utility of this approach. Figure 8 provides three reduced sensor sets with 32, 16, and five sensors respectively. The first sensor set contains measurements at all 96 degrees of freedom. Figure 9 contains the results for two damage cases. Figure 9a contains the results for damage case H. The appropriate locations were flagged in all cases. However, the actual element damaged was somewhat ambiguous as two elements were suggested as damaged. The case N results are generally considered impossible due to the low strain energy seen by this member. However, the coupled approach identifies the correct location for 96 measurements. The correct end of the structure is identified in case N. It should be noted that the results seen in Fig. 9 are a significant improvement over past results and represent one of the most significant technical developments associated with this project.

References
¹R. L. Schmitt, R. J. Williams, and J. D. Matthews. "High Frequency Scannerless Imaging Laser Radar for Industrial Inspection and Measurement Applications," SAND96-2739, Sandia National Laboratories, Albuquerque, NM, Nov. 1996.
²T. Cao and D. C. Zimmerman. "A Procedure to Extract Ritz Vectors from Dynamic Testing Data," ASCE J. of Earthquake Engineering. (Submitted for publication.)
³G. H. James, D. C. Zimmerman, and T. Cao. "Issues and Results Relating to the Calculation of Damage Models from Frequency Domain Data," 17th Int'l Modal Analysis Conf., Kissimmee, FL, Feb. 8-11, 1999. (To be presented.)
⁴T. Cao, D. C. Zimmerman, and G. H. James. "Identification of Ritz Vectors from Ambient Test Data," Proc. of the 16th Int'l Modal Analysis Conf., Santa Barbara, CA, Feb. 3-5. 1998.
⁵M. S. Grygier. "Modal Test Technology as Non-Destructive Evaluation of Space Shuttle Structures," Proc. of the Dual Use Space Technology Transfer Conf. and Exhibition, ed. K. Krishen, NASA CP-3263, 1994. 329-34.
⁶G. H. James, D. C. Zimmerman, and K. C. Chhipwadia. "Application of Autonomous Modal Identification to Traditional and Ambient Data Sets," 17th Int'l Modal Analysis Conf., Kissimmee, FL, Feb. 8-11, 1999. (To be presented.)
⁷D. C. Zimmerman, S. W. Smith, H.-M. Kim, and T. Bartkowicz. "An Experimental Study of Structural Damage Detection Using Incomplete Measurements," ASME J. of Vibration and Acoustics 118.4 (Oct. 1996): 543-50.
⁸D. C. Zimmerman and M. Kaouk. "Structural Damage Detection Using a Minimum Rank Update Theory," ASME J. of Vibration and Acoustics 116.2 (1994): 222-31.
⁹L. D. Peterson, S. W. Doebling, and K. F. Alvin. "Experimental Determination of Local Structural Stiffness by Disassembly of Measured Flexibility Matrices," Proc. of the AIAA Structures, Structural Dynamics, and Materials Conf., AIAA paper #95-1090, 1995.
¹⁰G. H. James, D. C. Zimmerman, and T. Cao. "Development of a Coupled Approach for Structural Damage Detection," Proc. of the 35th Aerospace Sciences Mtg. & Exhibit, Reno, NV, AIAA paper #97-0362, 1997; AIAA J. (Accepted for publication.)
¹¹T. A. L. Kashangaki. "Ground Vibration Tests of a High Fidelity Truss for Verification of On-Orbit Damage Location Techniques," NASA-LaRC Technical Memorandum 107626, NASA Langely Research Center, Langely, VA, 1992.

Publications
Abdallah, M. O., K. Grigoriadis, and D. C. Zimmerman. "Enhanced Damage Detection Using Linear Matrix Inequalities (LMI's)," Proc. of the SEM Int'l Modal Analysis Conf. XVI, Santa Barbara, CA, Feb. 1998.
Abdallah, M. O., K. Grigoriadis, and D. C. Zimmerman. "Enhanced Damage Detection Using Alternating Projections," 15th Int'l Modal Analysis Conf., Orlando, FL, Feb. 1997. 1325-31.
Abdallah, M., K. M. Grigoriadis, and D. C. Zimmerman. "Enhanced Structural Model Refinement and Damage Detection Using Alternating Projections," AIAA J. (Accepted for publication.)
Andre, G., C. Carrasco, R. Osegueda, C. Ferregut, G. H. James III, and M. Grygier. "Comparison of Accelerometer and Laser Modal Tests of a Vertical Stabilizer Assembly," Proc. of the 6th Int'l Conf. and Exhibition on Engineering, Construction, and Operations in Space, Albuquerque, NM, April 26-30, 1998.
Bartkowicz, T. J., H.-M. Kim, D. C. Zimmerman, and S. W. Smith. "Autonomous Structural Health Monitoring System: A Demonstration," Proc. of the 37th AIAA Structures, Structural Dynamics and Materials Conf., Salt Lake City, UT, April 1996. 1758-64.
Cao, T. T. and D. C. Zimmerman. "Application of Load Dependent Ritz Vectors in Structural Damage Detection," AIAA J. (Submitted for publication.)
Cao, T. T. and D. C. Zimmerman. "Effects of Noise on Measured Ritz Vectors," 1997 ASME Design Engineering Technical Conf., ASME Paper DETC97/VIB-4230.
Cao, T. T. and D. C. Zimmerman. "A Procedure to Extract Ritz Vectors from Dynamic Testing Data," ASCE J. of Structural Engineering. (Submitted for publication.)
Cao, T. and D. C. Zimmerman. "Application of Load Dependent Ritz Vectors in Structural Damage Detection," 15th Int'l Modal Analysis Conf., Orlando, FL, Feb. 1997. 1319-24.
Cao, T. and D. C. Zimmerman. "A Procedure to Extract Ritz Vectors from Dynamic Testing Data," 15th Int'l Modal Analysis Conf., Orlando, FL, Feb. 1997. 1036-42.
Cao, T. T., D. C. Zimmerman, and G. James. "Identification of Ritz Vectors from Ambient Test Data," Proc. of the SEM Int'l Modal Analysis Conf. XVI, Santa Barbara, CA, Feb. 1998.
Cao, T. T., D. C. Zimmerman, and G. H. James III. "Identification of Ritz Vectors from the Orbiter Vertical Stabilizer Assembly," SEM Int'l Modal Analysis Conf. XVII, Kissimmee, FL, Feb. 1999. (Accepted for publication.)
Chamitoff, G., G. H. James, III, and D. Barker. "Surviving on Mars without Nuclear Energy," Proc. of the Founding Convention of the Mars Soc., Boulder, CO, Aug. 13-16, 1998. (To appear.)
Chippwaddia, K. S., D. C. Zimmerman, and G. H. James III. "Evolving Autonomous Modal Parameter Estimation," SEM Int'l Modal Analysis Conf. XVII, Kissimmee, FL, Feb. 1999. (Accepted for publication.)
Gafka, G. and D. C. Zimmerman. "Structural Damage Detection via Least Squares Dynamic Residual Force Minimization with Quadratic Measurement Error Inequality Constraint," Proc. of the 14th Int'l Modal Analysis Conf., Dearborn, MI, Feb. 1996. 1278-84.
James, G., D. C. Zimmerman, and G. James. "Utilization of Large Experimental/Analytical Data Sets for Structural Health Monitoring of Aerospace Structures," 15th Int'l Modal Analysis Conf., Orlando, FL, Feb. 1997. 1765-71.
James III, G. H., D. C. Zimmerman, and T. Cao. "Development of a Coupled Approach for Structural Damage Detection with Incomplete Measurements," AIAA J. (Accepted for publication.)
James, G., D. C. Zimmerman, and T. W. Simmermacher. "Reduction/Expansion Studies for Damage Identification of Continuous Aerospace Structures," 15th Int'l Modal Analysis Conf., Orlando, FL, Feb. 1997. 1772-78.
James, G. H., D. C. Zimmerman, and M. Grygier. "An Experimental Database for Model Correlation and Damage Detection Studies Based on the Space Shuttle Vertical Stabilizer Assembly (VSA) Test Article," Proc. of the SEM Int'l Modal Analysis Conf. XVI, Santa Barbara, CA, Feb. 1998.
James, G. H., D. C. Zimmerman, and R. L. Mayes. "An Experimental Study of Frequency Response Function (FRF) Based Damage Assessment Tools," Proc. of the SEM Int'l Modal Analysis Conf. XVI, Santa Barbara, CA, Feb. 1998.
James, III, G. H., D. C. Zimmerman, and T. T. Cao. "Application of Autonomous Modal Identification to Traditional and Ambient Data Sets," SEM Int'l Modal Analysis Conf. XVII, Kissimmee, FL, Feb. 1999. (Accepted for publication.)
James, III, G.H., D. C. Zimmerman, T. T. Cao, and R. L. Mayes. "Issues and Results Relating to the Calculation of Damage Models from Frequency Domain Data," SEM Int'l Modal Analysis Conf. XVII, Kissimmee, FL, Feb. 1999. (Accepted for publication.)
Kaouk, M. and D. C. Zimmerman. "Assessment of Damage Affecting all Structural Properties Using Experimental Modal Parameters," ASME J. of Vibration and Acoustics. (Accepted for publication.)
Kaouk, M. and D. C. Zimmerman. "Reducing the Required Number of Modes for Structural Damage Assessment," Proc. of the 36th AIAA Structures, Structural Dynamics and Materials Conf., New Orleans, LA, April 1995. 2802-12.
James, III, G. H., G. Chamitoff, and D. Barker. "Resource Utilization and Site Selection for a Self-Sufficient Martian Outpost," NASA/TM-98-206538, NASA-JSC, April 1998.
James, III, G. H., T. G. Carne, and P. S. Veers. "Damping Measurements Using Operational Data," ASME J. of Solar Energy Engineering 18.3 (Aug. 1996).
Kaouk, M. and D. C. Zimmerman. "Structural Health Assessment Using A Partition Model Update Technique," 13th Int'l Modal Analysis Conf., Nashville, TN, Feb. 1995. 1673-79.
Larson, C. B., D. C. Zimmerman, and E. L. Marek. "A Comparative Study of Metrics for Modal Pre-Test Sensor and Actuator Selection," AIAA J. (Submitted for publication.)
Malcolm, D. J. and G. H. James III. "Stability of a 26m Teetered, Free-Yaw Wind Machine," in Wind Energy 1996. Eds. W. Musial, S. Hock, and D. Berg, New York: ASME, SED-15: 1996.
Pappa, R. S., G. H. James III, and D. C. Zimmerman. "Autonomous Modal Identification of the Space Shuttle Tail Rudder," AIAA J. of Spacecraft and Rockets 35.2 (March-April 1998): 163-69; ASME Paper DETC97/VIB-4250.
Santos, J. M. C. D. and D. C. Zimmerman. "Damage Detection in Complex Structures Using Component Mode Synthesis and Residual Modal Force Vectors," Proc. of the 14th Int'l Modal Analysis Conf., Dearborn, MI, Feb. 1996. 1299-05.
Santos, J. M. C. D. and D. C. Zimmerman. "Damage Detection Using Minimum Rank Update Theory and Parameter Estimation," Proc. of the AIAA/ASME Adaptive Structures Forum, April 1996, Salt Lake City, UT. 168-75.
Simmermacher, T. and D. C. Zimmerman. "On the Detection of Damage in Bridges," ASCE J. of Structural Engineering. (Submitted for publication.)
Simmermacher, T. and D. C. Zimmerman. "Model Refinement and Damage Detection Using Nearfield Acoustic Holography" 15th Int'l Modal Analysis Conf., Orlando, FL, Feb. 1997. 1758-64.; J. of the Acoustical Soc. of America. (Submitted for publication.)
Simmermacher, T., M. Kaouk, and D. C. Zimmerman. "Exploiting Aliasing Effects in the Eigensystem Realization Algorithm," 15th Int'l Modal Analysis Conf., Orlando, FL, Feb. 1997. 50-63.
Simmermacher, T., D. C. Zimmerman, R. Mayes, G. Reese, and G. James. "The Effects of Finite Element Grid Density on Model Correlation and Damage Detection of a Bridge," Proc. of the 36th AIAA Structures, Structural Dynamics and Materials Conf., New Orleans, LA, April, 1995. 2249-58.
Smith, S. W., D. C. Zimmerman, H.-M. Kim, and T. J. Bartkowicz. "Damage Detection in Damped Structures," 15th Int'l Modal Analysis Conf., Orlando, FL, Feb. 1997. 1096-1102.
Smith, S. W., D. C. Zimmerman, T. J. Bartkowicz, and H.-M. Kim. "Experiments for Damage Location in a Damped Structure," ASME Paper DETC97/VIB-4231.
Smith, S. W., J. C. Eckert, and D. C. Zimmerman. "Zero-Gravity Damage Evaluation (Z-GraDE)," Proc. of the SEM Int'l Modal Analysis Conf. XVI, Santa Barbara, CA, Feb. 1998.
Yap, K. C. and D. C. Zimmerman. "A Comparison of Structural Dynamic Modification and Sensitivity Method Approximations," Proc. of the SEM Int'l Modal Analysis Conf. XVI, Santa Barbara, CA, Feb. 1998.
Yap, K. C. and D. C. Zimmerman. "The Effect of Coding in Genetic Algorithm Based Structural Damage Detection," Proc. of the SEM Int'l Modal Analysis Conf. XVI, Santa Barbara, CA, Feb. 1998.
Zimmerman, D. C. and G. H. James III. "Statistical Confidence Using Minimum Rank Perturbation Theory," Proc. of the SEM Int'l Modal Analysis Conf. XVI, Santa Barbara, CA, Feb. 1998.
Zimmerman, D. C. and T. Simmermacher. "Structural Health Monitoring Using Vibration Measurements and Engineering Insight," ASME Design Engineering Division Special Anniv. Issue 115 (June 1995): 214-21.
Zimmerman, D. C. and T. Simmermacher. "Model Correlation Using Multiple Static Load and Vibration Tests," AIAA J. 33.11 (1995): 2182-88.
Zimmerman, D. C., S. W. Smith, H.-M. Kim, and T. Bartkowicz. "An Experimental Study of Structural Damage Detection Using Incomplete Measurements," ASME J. of Vibration and Acoustics 118.4 (Oct. 1996): 543-50.
Zimmerman, D. C. and M. Kaouk. "Structural Health Assessment Using a Partition and Element Model Update Technique," J. of Chinese Soc. of Mechanical Engineers 19.1 (Feb. 1998).
Zimmerman, D. C. and D. S. Layton. "Large Angle Slewing Maneuvers Using Performance Driven Darwinian Learning Controllers: Theory and Experiment," AIAA J. of Guidance, Control and Dynamics. (Accepted for publication.)
Zimmerman, D. C., G. H. James, and T. T. Cao. "Recent Structural Health Monitoring Technologies with Spacecraft Component Experimental Validation," Proc. of the SEM Spring Mtg., Houston, TX, June 1998. 114-17.
Zimmerman, D. C., G. H. James III, and T. T. Cao. "An Experimental Study of Damage Detection Using Modal and Ritz Properties," SEM Int'l Modal Analysis Conf. XVII, Kissimmee, FL, Feb. 1999. (Accepted for publication.)
Zimmerman, D. C., T. Simmermacher, and M. Kaouk. "Model Correlation and System Health Monitoring Using Frequency Domain Measurements," ASME J. of Vibration and Acoustics. (Submitted for publication.)
Zimmerman, D. C., C. K. Yap, and T. Hasselman. "Evolutionary Approach for Model Correlation," Mechanical Systems and Signal Processing. (Submitted for publication.)
Zimmerman, D. C., T. Simmermacher, and M. Kaouk. "Model Correlation And System Health Monitoring Using Frequency Domain Measurements," Proc. of the AIAA/ASME Adaptive Structures Forum, New Orleans, LA, April 1995. 3318-26.
Zimmerman, D. C., H.-M. Kim, T. J. Bartkowicz, and M. Kaouk. "Damage Detection Using Expanded Dynamic Residuals," Proc. of the AIAA Structures, Structural Dynamics and Materials Conf., April 1998. (To be published.)
Zimmerman, D. C., T. Simmermacher, and M. Kaouk. "On the Role of Engineering Insight & Judgement in Structural Damage Detection," 13th Int'l Modal Analysis Conf., Nashville, TN, Feb. 1995. 179-84.
Zimmerman, D. C., M. Kaouk, and T. Simmermacher. "Structural Damage Detection Using Frequency Response Functions," 13th Int'l Modal Analysis Conf., Nashville, TN, Feb. 1995. 414-20.
Zimmerman, D. C., S. W. Smith, H.-M. Kim, and T. J. Bartkowicz. "Spacecraft Applications for Damage Detection Using Vibration Testing," Proc. of the 14th Int'l Modal Analysis Conf., Dearborn, MI, Feb. 1996. 851-56.
Zimmerman, D. C., Yap, C.K., and Hasselman, T., "Evolutionary Approach for Model Correlation," 15th Int'l Modal Analysis Conf., Orlando, FL, Feb. 1997. 551-57.
Presentations
Cao, T. T., D. C. Zimmerman, and G. James. "Identification of Ritz Vectors From Ambient Test Data," SEM Int'l Modal Analysis Conf. XVI, Santa Barbara, CA, Feb. 2-5, 1998.
James, G. H. "Damage Detection-Building/Earthquakes," chair, a technical session at the 16th Int'l Modal Analysis Conf., Santa Barbara, CA, Feb. 3-5. 1998.
James, G. H. "Technology Presentations of Structural Health Monitoring Products and Services," developer, a technical session at the 1998 SEM Spring Conf. and Exhibition, Houston, TX, June 1, 1998.
James, G., D. C. Zimmerman, and G. James. "Utilization of Large Experimental/Analytical Data Sets for Structural Health Monitoring of Aerospace Structures," 15th Int'l Modal Analysis Conf., Orlando, FL, Feb. 1997.
James, G., D. C. Zimmerman, and T. T. Cao. "Development of a Coupled Approach for Structural Damage Detection," Proc. of the 35 Aerospace Sciences Mtg. & Exhibit, Reno, NV, Jan. 1997.
James, G., D. C. Zimmerman, and T. W. Simmermacher. "Reduction/Expansion Studies for Damage Identification of Continuous Aerospace Structures," 15th Int'l Modal Analysis Conf., Orlando, FL, Feb. 1997.
James, G. H., D. C. Zimmerman, and M. Grygier. "An Experimental Database for Model Correlation and Damage Detection Studies Based on the Space Shuttle Vertical Stabilizer Assembly (VSA) Test Article," SEM Int'l Modal Analysis Conf. XVI, Santa Barbara, CA, Feb. 2-5, 1998.
James, G. H., D. C. Zimmerman, and R. L. Mayes. "An Experimental Study of Frequency Response Function (FRF) Based Damage Assessment Tools," SEM Int'l Modal Analysis Conf. XVI, Santa Barbara, CA, Feb. 2-5, 1998.
Zimmerman, D. C. and G. H. James, III. "Statistical Confidence Using Minimum Rank Perturbation Theory," SEM Int'l Modal Analysis Conf. XVI, Santa Barbara, CA, Feb. 2-5, 1998.
Zimmerman, D. C., G. H. James, and T. T. Cao. "Recent Structural Health Monitoring Technologies with Spacecraft Component Experimental Validation," special two-hour presentation allocated by the SEM Spring Mtg. Organizers, SEM Spring Mtg., Houston, TX, June 1998.
Zimmerman, D. C., G. James, C. Farrar, and S. Doebling. "Current Horizon for Damage Detection," three day short course developed and presented at the invitation of the Soc. of Experimental Mechanics (SEM) before the 15th Int'l Modal Analysis Conf. Thirty professionals attended the short course, including personnel from the Federal Highway Admin., Sandia Nat'l Laboratories, Nat'l Renewable Energy Laboratory, EMPA-Swiss Federal Lab for Materials Testing, US Army White Sands Missile Range, Argonne Nat'l Laboratory, Alcoa, Kubota Corp. (Japan), NKK Corp. (Japan), and Intevep, S.A. (Venezuela). Based on the interest and feedback from the participants, the SEM has asked us to offer this short course at the 17th Int'l Modal Analysis Conf. in February 1999.
Funding
"Efficient Model Reduction Methods for Large Scale Multibody Systems," with K. Grigoriadis, Dynacs Engineering Co.; graduate student stipend.
"Experimental Analysis Using a Laser Dynamic Range Imager (LDRI) Sensor." NASA-JSC, May 1, 1998-April 30, 1999.
"Experimental Identification of Ritz Properties From Dynamic Testing." National Science Foundation, Aug. 24, 1998-Aug. 23, 1999, $64,783.
"Integrated Software for Improving the Design and Performance of Advanced Aerospace Structures-- Phase II." NASA-LRC, Jan. 1, 1997-Dec. 31, 1998; subcontract from ACTA, Inc.
"Mir Structural Dynamics Experiment (MiSDE) Flight Project." NASA-JSC, Oct. 1, 1996-March 31, 1998, $35,000; subcontract from McDonnell Douglas Aerospace.
"A Preliminary Investigation of the Cannon Wind Eagle 300 Rotor Subassembly Joint Modeling And Updating with Test Data." National Renewable Energy Laboratory, Nov. 1997-Nov. 1998, $7,376.
"Technical Support for Structural Health Monitoring, Model Correlation, and Ambient Vibration Analysis." Sandia Nat'l Laboratories, Nov. 1, 1996-Sept. 30, 1998, $52,883.
"ZGRADE--Zero Gravity Damage Experiment." NASA Reduced Gravity Office, Dec. 1996-April, 1997; access granted to NASA KC-135 Facility including 6-8 hours of actual flight time (80 parabolas).

Contents
ISSO -- Institute for Space Systems Operations
1997-1998 Annual Report